It has long been known that a wide range of surfaces on aircraft, automobiles, ship hulls, oil drilling rigs, water intakes in power plants, and the like could benefit from engineered coatings that minimize drag and adhesion of a variety of substances such as insect residue, dirt, ice, bio-organisms, deposits such as mineral deposits, etc. For example, future designs of aircraft want to take advantage of laminar flow to improve fuel efficiency. Studies have shown that for long haul flights, increases in fuel efficiency as high as 12% are possible with hybrid laminar flow control and natural laminar flow (Kirchner, M. E. NASA CP-2487, 1987, Part 1, pp. 24-44). In order to maintain laminar flow operationally, however, the aerodynamic surfaces must be relatively smooth and not allow accumulation of any substances that can interrupt laminar flow, such as insect residue or ice. In particular, a number of approaches have been attempted to reduce insect residue adhesion to aircraft surfaces such as, mechanical scrapers, sacrificial coatings or covers, and continual wetting of the surfaces during take-off and landing. All of these suffer from problems such as adding significant weight, complexity or were simply impractical in the industry (Coleman, W. S. “Boundary Layer and Flow Control”, ed. G. V. Lachman, Pergamon Press, 1961, pp. 682-747. Lachman, G. V. Ministry of Aviation Aeronautical Research Council, A.R.C. Technical Report, 1960, C.P. No. 484). Passive strategies for minimizing fouling or contamination of surfaces are beneficial especially in environments where active mitigation of the fouling or contamination is impractical or impossible. One approach with promise has been the use of coatings, however, no coatings developed to date have been able to satisfy all of the requirements needed for laminar flow maintenance. Such an approach could involve modification of a material's surface energy either chemically or topographically or by using combinations thereof.
Any surface material needs to meet the requirements of its application. High performance polymeric materials have been developed to address various requirements for mechanical, thermal, and optical properties. Modification of the chemical constituency of these polymeric materials can alter their properties. Often there is a trade-off, for example, increasing the stiffness or modulus of a polymeric material typically comes with a sacrifice in toughness. Thus, modification of high performance polymeric materials is often hampered due to degradation of the desired characteristic properties. Modifying a polymeric material to change properties of the surface is problematic as addition of sufficient modifier to the bulk chemical composition to achieve the desired surface modification could also result in the diminution of important properties of the polymeric material. If the modifier is well dispersed within the polymer matrix, a majority of the modifier will be contained within the interior of the polymeric structure and will not contribute to modification of the polymer or coating surface. This is of greater consequence if the modifier is expensive, provides no advantage, or diminishes bulk properties. Polymeric materials with low adhesion surface properties have been demonstrated to be effective in a wide variety of applications.
Low surface energy polymeric materials, i.e., those exhibiting a high water contact angle, have been used to reduce biofouling, water and ice adhesion, and biofilm formation; to improve oxidation, corrosion and stain resistance; to minimize dust adhesion; and to modify the performance of microfluidic systems and biomedical devices. The ability to selectively modify the surface energy of high performance polymeric materials without sacrificing their superior mechanical, thermal, or optical properties would be of significant utility.
A number of approaches have been suggested to provide polymeric materials with low surface energy. Some of the most well-known polymeric materials having low surface energy are fluorinated, aliphatic polymers such as those available under the trade name TEFLON®. The presence of both aliphatic carbon species and fluorine atoms contributes to the low surface energy of this class of materials. These polymeric materials have an approximate homogeneous composition. These polymeric materials do not use a controlled modification and thus cannot be readily tailored for the introduction of further surface features. Moreover, they are difficult to adhere to substrates, and generally the polymer is available only as a powder and must be sintered or melted to coat the desired surface. With these difficulties in coating surfaces, delamination can become an issue during use. Another approach is to vapor deposit highly fluorinated carbonaceous materials to various substrates.
Another approach to provide low surface energy polymeric materials is to incorporate surface modifying agents into the materials. These surface modifying agents are thermodynamically driven to the surface of the polymeric material due to more favorable interactions at the air interface as compared with interactions within the bulk polymeric matrix.
Fluorine-containing oxetane derivatives have been used extensively as surface modification agents for modification of urethanes. See, for instance, Malik, et al., United States patent application publication No. US 2004/0087759. Omnova Solutions Inc. offers a family of hydroxyl terminated oxetane-derived oligomers under the trade name POLYFOX® fluorochemicals.
Epoxy is a term used for both the basic component and the cured end product of epoxy resins, as well as a generic name for the epoxide functional group. Epoxy resins, also known as polyepoxides, are a class of reactive monomers, prepolymers and polymers which contain epoxide groups. Epoxy resins may be reacted (cross-linked) either with themselves through catalytic homopolymerization, or with a wide range of co-reactants including polyfunctional amines, acids, anhydrides, phenols, alcohols, and thiols. These co-reactants are often referred to as hardeners or curatives, and the cross-linking reaction is commonly referred to as curing. Reaction of polyepoxides with themselves or with polyfunctional hardeners forms a thermosetting polymer, often with high mechanical properties and temperature and chemical resistance. The range of chemistry and property combinations in epoxies is extensive, consequently a diverse array of epoxy formulations are available. Thus they have a wide range of applications such as coatings and paints, fiber-reinforced composites, and functional and structural adhesives.
Epoxy resins are among the most important industrial polymers in the world and are used in large quantities in the production of adhesives, paints and coatings, and matrix resins. The core substrate in the production of epoxy resins may include 2,2-bis(4-hydroxyphenyl)isopropylidiene (bisphenol A). The main monomer used in the epoxy resin industry is the diglycidyl ether of bisphenol A, 2,2-Bis(4-glycidyloxyphenyl)propane (DGEBA), which represents more than 75% of the resin used in industrial applications. 2,2-Bis(4-glycidyloxyphenyl)propane is usually prepared from 2,2-Bis(4-hydroxyphenyl)isopropylidiene (bisphenol A) and epichlorohydrin using a strong base such as sodium hydroxide ether. DGEBA resins normally contain some distribution of molecular weight and exhibit a viscosity in the range of 5-15 Pascal-second at 25° C. Alternative synthetic methods to prepare DGEBA have been developed such as allylating bisphenol A followed by epoxidization.
Another common epoxy is N,N,N′,N′-tetraglycidyl-4,4′-methylenedianiline (TGMDA) prepared from 4,4′-methylenedianiline and epichlorohydrin using a strong base. TGMDA epoxies are characterized by high cross-link densities, which results in a high modulus of elasticity and a high glass transition temperature. Aerospace structural epoxy matrix resins are typically based on TGMDA.
Accordingly, a desire exists to provide a polymeric material that has the mechanical, thermal, chemical, and optical properties associated with epoxies yet achieve a low energy surface.